The quantity of respirable aerosol particles is an important factor in the determination of the inhalation hazards of aerosol particles. Primate and animal nasal passages have structures and hair that allow inertial mechanisms to remove aerosol particles from the air moving in these passages. The aerodynamic diameter size distribution of aerosol particles in the nasal passage is determinative of the quantity of respirable aerosol particles. The relationship between the percentage of penetration of the aerosol particles to the respiratory tract and the particle aerodynamic diameter has been determined by several organizations such as the British Medical Research Council (BMRC), the U.S. Atomic Energy Commission (AEC), and the American Conference of Governmental Industrial Hygienists (ACGIH). These relationships are represented in the graph of FIG. 1. FIG. 1 indicates that the penetration to the respiratory tract decreases from about 100% at 2.mu. to about 0% at 10.mu. aerodynamic diameter.
The quantity of respirable particles in an aerosol can be found from one of two general methods. One method is to measure the particle size distribution of the entire aerosol, and then apply one of the penetration curves of FIG. 1 to the resultant size distribution. The other method is to pass the aerosol through a size classification device which has a penetration curve similar to one of the curves in FIG. 1, and analyze the particles which penetrate the classification device.
Two size classification devices have been commonly used to approximate the respirable penetration curves. These devices are the horizontal elutriator and the cyclone. The penetration characteristics of the elutriator is determined by physical theory. The penetration characteristics of the cyclone is determined experimentally.
Another sampling device which classifies particles by their aerodynamic size is the inertial impactor. Impactors have been used as respirable preseparators. They have not found wide acceptance because their cut-off characteristics are much sharper than the respirable penetration curves. For example, the single cut-off size impactor collector efficiency is much steeper than the ACGIH curve as shown in FIG. 19. An advantage of an impactor is that the particle collection characteristics can be predicted. They are also compact in structure, operate in any position, and are simple to construct.
Theoretical analysis techniques have been developed so that the fluid flow field and the characteristic particle collection efficiency curve, or penetration curve, of inertial impactors of both round and rectangular nozzles can be accurately predicted. Experimental investigations have shown that these theoretically predicted collection efficiency curves agree well with experimentally determined efficiency curves.
The 50% particle cut-off size D.sub.P.sbsb.50, of an impactor (i.e., the particle size at which 50% are collected and 50% penetrate) is governed by a particle collection criteria as represented in the following equation: ##EQU1## where .mu. = absolute air viscosity
W = nozzle diameter (round impactor) or nozzle width (rectangular impactor) PA1 Stk.sub.50 = Stokes number corresponding to 50% particle collection PA1 C = cunningham slip correction PA1 V.sub.o = average velocity in nozzle = flow rate/nozzle area
Variations of the components of equation (1) can be made without substantially altering particle collection characteristics.
The value of Stk.sub.50 is a function of the Reynolds number of the flow in the impactor nozzle, Re, and of the dimensionless parameters S/W and T/W, where S = jet-to-plate distance and T = throat length. The influence of these three parameters on the value of Stk.sub.50 has previously been published. "Characteristics of Laminar Jet Impactors", Marple and Liu, Environmental Science and Technology, Vol. 8, No. 7, pp. 648-654, July 1974. This work shows that if the impactor is designed such that S/W &gt; 1.0 (round impactor) or S/W &gt; 1.5 (rectangular impactor) and T/W &gt; 1/4, their influence on D.sub.P.sbsb.50 will be small. Thus, the Reynolds number, defined as ##EQU2## where .rho. = the air density, will then be the major parameter in defining the value of Stk.sub.50 and the shape of the collection efficiency curve.
Another consideration in the analysis of impactors is that the pressure drops, .DELTA.P, across the impactor nozzle is approximately equal to the dynamic pressure of the air jet in the nozzle, EQU .DELTA.P = 1/2 .rho. V.sub.o.sup.2 ( 4)
Additional reference is made to the publication "On Fluid Flow and Aerosol Impaction in Inertial Impactors", Journal of Colloidal and Interface Science, Vol. 53, No. 1, October 1975, pp. 31-34, Marple and Liu.
High volume sampling apparatus and personal samplers are used for sampling of suspended particles in outdoor and occupational environments. Air pollution and occupational health monitoring are concerned with aerosol particles which size is within the respirable range, i.e., less than 8 to 10 microns. Aerosol particles of this size upon inhalation by humans enter and are retained by the respiratory tract and lungs. Particles larger than the respirable range generally do not constitute a health hazard, although they may cause soiling, corrosion, and other adverse environmental effects. The standard high volume air sampler collects essentially all particles which reach its surface. These particles fall within the size range from about 0 to 100 microns. This sampler does not per se permit an analysis of the respirable particles. Cascade impactors are used with this type of sampler to obtain particle size distribution data. An example of this impactor is disclosed by Anderson in U.S. Pat. Nos. 3,011,914 and 3,795,135.